Thenitrogen cycle is thebiogeochemical cycle by whichnitrogen is converted into multiple chemical forms as it circulates amongatmospheric,terrestrial, andmarine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle includefixation,ammonification,nitrification, anddenitrification. The majority ofEarth's atmosphere (78%) is atmosphericnitrogen,[16] making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to ascarcity of usable nitrogen in many types ofecosystems.
The nitrogen cycle is of particular interest toecologists because nitrogen availability can affect the rate of key ecosystem processes, includingprimary production anddecomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramaticallyaltered the global nitrogen cycle.[17][18][19] Human modification of the global nitrogen cycle can negatively affect the natural environment system and also human health.[20][21]
| Part of a series on |
| Biogeochemical cycles |
|---|
 |
Nitrogen is present in the environment in a wide variety of chemical forms including organic nitrogen,ammonium (NH+4),nitrite (NO−2),nitrate (NO−3),nitrous oxide (N2O),nitric oxide (NO) or inorganic nitrogen gas (N2). Organic nitrogen may be in the form of a living organism,humus or in the intermediate products of organic matter decomposition. The processes in the nitrogen cycle is to transform nitrogen from one form to another. Many of those processes are carried out bymicrobes, either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth. For example, thenitrogenous wastes in animalurine are broken down bynitrifying bacteria in the soil to be used by plants. The diagram alongside shows how these processes fit together to form the nitrogen cycle.
The conversion of nitrogen gas (N2) into nitrates and nitrites through atmospheric, industrial and biological processes is called nitrogen fixation. Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants. Between 5 and 10 billion kg per year are fixed bylightning strikes, but most fixation is done by free-living orsymbioticbacteria known asdiazotrophs. These bacteria have thenitrogenaseenzyme that combines gaseous nitrogen withhydrogen to produceammonia, which is converted by the bacteria into otherorganic compounds. Most biological nitrogen fixation occurs by the activity ofmolybdenum (Mo)-nitrogenase, found in a wide variety of bacteria and someArchaea. Mo-nitrogenase is a complex two-componentenzyme that has multiple metal-containing prosthetic groups.[22] An example of free-living bacteria isAzotobacter. Symbiotic nitrogen-fixing bacteria such asRhizobium usually live in the root nodules oflegumes (such as peas, alfalfa, and locust trees). Here they form amutualistic relationship with the plant, producing ammonia in exchange forcarbohydrates. Because of this relationship, legumes will often increase the nitrogen content of nitrogen-poor soils. A few non-legumes can also form suchsymbioses. Today, about 30% of the total fixed nitrogen is produced industrially using theHaber-Bosch process,[23] which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia.[24]
Plants can absorb nitrate or ammonium from the soil by their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll. In plants that have a symbiotic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. It is now known that there is a more complex cycling of amino acids betweenRhizobia bacteroids and plants. The plant provides amino acids to the bacteroids so ammonia assimilation is not required and the bacteroids pass amino acids (with the newly fixed nitrogen) back to the plant, thus forming an interdependent relationship.[25] While many animals, fungi, and otherheterotrophic organisms obtain nitrogen by ingestion ofamino acids,nucleotides, and other small organic molecules, other heterotrophs (including manybacteria) are able to utilize inorganic compounds, such as ammonium as sole N sources. Utilization of various N sources is carefully regulated in all organisms.
When a plant or animal dies or an animal expels waste, the initial form of nitrogen isorganic, present in forms such as amino acids and DNA.[26] Bacteria and fungi convert this organic nitrogen intoammonia and sometimes ammonium through a series of processes called ammonification ormineralization. This is the last step in the nitrogen cycle step involving organic compounds.[27] Myriad enzymes are involved includingdehydrogenases,proteases, anddeaminases such asglutamate dehydrogenase andglutamine synthetase.[28] Nitrogen mineralization and ammonification have a positive correlation with organic nitrogen in the soil,[29] soil microbial biomass, and average annual precipitation.[30] They also respond closely to changes in temperature.[31] However, these processes slow in the presence of vegetation with high carbon to nitrogen ratios[32][33] and fertilization with sugar.[34][35]
The conversion of ammonium to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria. In the primary stage of nitrification, the oxidation of ammonium (NH+4) is performed by bacteria such as theNitrosomonas species, which converts ammonia tonitrites (NO−2). Other bacterial species such asNitrobacter, are responsible for the oxidation of the nitrites (NO−2) intonitrates (NO−3). It is important for theammonia (NH3) to be converted to nitrates or nitrites because ammonia gas is toxic to plants.
Due to their very highsolubility and because soils are highly unable to retainanions, nitrates can entergroundwater. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and causemethemoglobinemia or blue-baby syndrome.[38] Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute toeutrophication, a process that leads to high algal population and growth, especially blue-green algal populations. While not directly toxic to fish life, like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies. Since 2006, the application of nitrogenfertilizer has been increasingly controlled in Britain and the United States. This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies.
Denitrification is the reduction of nitrates back into nitrogen gas (N
2), completing the nitrogen cycle. This process is performed by bacterial species such asPseudomonas andParacoccus, under anaerobic conditions. They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively (meaning optionally) anaerobic bacteria can also live in aerobic conditions. Denitrification happens in anaerobic conditions e.g. waterlogged soils. The denitrifying bacteria use nitrates in the soil to carry out respiration and consequently produce nitrogen gas, which is inert and unavailable to plants. Denitrification occurs in free-living microorganisms as well as obligate symbionts of anaerobic ciliates.[39]
Dissimilatory nitrate reduction to ammonium (DNRA), or nitrate/nitrite ammonification, is ananaerobic respiration process. Microbes which undertake DNRA oxidise organic matter and use nitrate as an electron acceptor, reducing it tonitrite, thenammonium (NO−3 → NO−2 → NH+4).[40] Both denitrifying and nitrate ammonification bacteria will be competing for nitrate in the environment, although DNRA acts to conserve bioavailable nitrogen as soluble ammonium rather than producing dinitrogen gas.[41]
TheANaerobicAMMoniaOXidation process is also known as theANAMMOX process, an abbreviation coined by joining the firstsyllables of each of these three words. This biological process is aredoxcomproportionation reaction, in whichammonia (thereducing agent giving electrons) andnitrite (theoxidizing agent accepting electrons) transfer threeelectrons and are converted into one molecule ofdiatomicnitrogen (N
2) gas and two water molecules. This process makes up a major proportion of nitrogen conversion in theoceans. Thestoichiometrically balanced formula for the ANAMMOX chemical reaction can be written as following, where anammoniumion includes the ammonia molecule, itsconjugatedbase:
This anexergonic process (here also anexothermic reaction) releasing energy, as indicated by the negative value of ΔG°, the difference inGibbs free energy between the products of reaction and the reagents.
Though nitrogen fixation is the primary source of plant-available nitrogen in mostecosystems, in areas with nitrogen-richbedrock, the breakdown of this rock also serves as a nitrogen source.[43][44][45] Nitrate reduction is also part of theiron cycle, under anoxic conditions Fe(II) can donate an electron toNO−3 and is oxidized to Fe(III) whileNO−3 is reduced toNO−2, N2O, N2, andNH+4 depending on the conditions and microbial species involved.[46] Thefecal plumes of cetaceans also act as a junction in the marine nitrogen cycle, concentrating nitrogen in the epipelagic zones of ocean environments before its dispersion through various marine layers, ultimately enhancing oceanic primary productivity.[47]
The nitrogen cycle is an important process in the ocean as well. While the overall cycle is similar, there are different players[50] and modes of transfer for nitrogen in the ocean. Nitrogen enters the water through the precipitation, runoff, or asN
2 from the atmosphere. Nitrogen cannot be utilized byphytoplankton asN
2 so it must undergo nitrogen fixation which is performed predominately bycyanobacteria.[51] Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 years.[52] Phytoplankton need nitrogen in biologically available forms for the initial synthesis of organic matter. Ammonia and urea are released into the water by excretion from plankton. Nitrogen sources are removed from theeuphotic zone by the downward movement of the organic matter. This can occur from sinking of phytoplankton, vertical mixing, or sinking of waste of vertical migrators. The sinking results in ammonia being introduced at lower depths below the euphotic zone. Bacteria are able to convert ammonia to nitrite and nitrate but they are inhibited by light so this must occur below the euphotic zone.[53] Ammonification orMineralization is performed by bacteria to convert organic nitrogen to ammonia.Nitrification can then occur to convert the ammonium to nitrite and nitrate.[54] Nitrate can be returned to the euphotic zone by vertical mixing and upwelling where it can be taken up by phytoplankton to continue the cycle.N
2 can be returned to the atmosphere throughdenitrification.
Ammonium is thought to be the preferred source of fixed nitrogen for phytoplankton because its assimilation does not involve aredox reaction and therefore requires little energy. Nitrate requires a redox reaction for assimilation but is more abundant so most phytoplankton have adapted to have the enzymes necessary to undertake this reduction (nitrate reductase). There are a few notable and well-known exceptions that include mostProchlorococcus and someSynechococcus that can only take up nitrogen as ammonium.[52]
The nutrients in the ocean are not uniformly distributed. Areas of upwelling provide supplies of nitrogen from below the euphotic zone. Coastal zones provide nitrogen from runoff and upwelling occurs readily along the coast. However, the rate at which nitrogen can be taken up by phytoplankton is decreased inoligotrophic waters year-round and temperate water in the summer resulting in lower primary production.[55] The distribution of the different forms of nitrogen varies throughout the oceans as well.
Nitrate is depleted in near-surface water except in upwelling regions. Coastal upwelling regions usually have high nitrate andchlorophyll levels as a result of the increased production. However, there are regions of high surface nitrate but low chlorophyll that are referred to asHNLC (high nitrogen, low chlorophyll) regions. The best explanation for HNLC regions relates to iron scarcity in the ocean, which may play an important part in ocean dynamics and nutrient cycles. The input of iron varies by region and is delivered to the ocean by dust (fromdust storms) and leached out of rocks. Iron is under consideration as the true limiting element to ecosystem productivity in the ocean.
Ammonium and nitrite show a maximum concentration at 50–80 m (lower end of theeuphotic zone) with decreasing concentration below that depth. This distribution can be accounted for by the fact that nitrite and ammonium are intermediate species. They are both rapidly produced and consumed through the water column.[52] The amount of ammonium in the ocean is about 3 orders of magnitude less than nitrate.[52] Between ammonium, nitrite, and nitrate, nitrite has the fastest turnover rate. It can be produced during nitrate assimilation, nitrification, and denitrification; however, it is immediately consumed again.
Nitrogen entering the euphotic zone is referred to as new nitrogen because it is newly arrived from outside the productive layer.[51] The new nitrogen can come from below the euphotic zone or from outside sources. Outside sources are upwelling from deep water and nitrogen fixation. If the organic matter is eaten, respired, delivered to the water as ammonia, and re-incorporated into organic matter by phytoplankton it is considered recycled/regenerated production.
New production is an important component of the marine environment. One reason is that only continual input of new nitrogen can determine the total capacity of the ocean to produce a sustainable fish harvest.[55] Harvesting fish from regenerated nitrogen areas will lead to a decrease in nitrogen and therefore a decrease in primary production. This will have a negative effect on the system. However, if fish are harvested from areas of new nitrogen the nitrogen will be replenished.
As illustrated by the diagram on the right, additionalcarbon dioxide (CO2) is absorbed by theocean and reacts with water,carbonic acid (H
2CO
3) is formed and broken down into bothbicarbonate (HCO−3) and hydrogen (H+
) ions (gray arrow), which reduces bioavailablecarbonate (CO2−3) and decreases oceanpH (black arrow). This is likely to enhance nitrogen fixation bydiazotrophs (gray arrow), which utilizeH+
ions to convert nitrogen into bioavailable forms such asammonia (NH
3) andammonium ions (NH+4). However, as pH decreases, and more ammonia is converted to ammonium ions (gray arrow), there is lessoxidation of ammonia tonitrite (NO–
2), resulting in an overall decrease in nitrification and denitrification (black arrows). This in turn would lead to a further build-up of fixed nitrogen in the ocean, with the potential consequence ofeutrophication. Gray arrows represent an increase while black arrows represent a decrease in the associated process.[49]
.jpg)
.jpg)
As a result of extensive cultivation of legumes (particularlysoy,alfalfa, andclover), growing use of theHaber–Bosch process in the production of chemicalfertilizers, and pollution emitted by vehicles and industrial plants, human beings have more than doubled the annual transfer of nitrogen into biologically available forms.[38] In addition, humans have significantly contributed to the transfer of nitrogen trace gases from Earth to the atmosphere and from the land to aquatic systems. Human alterations to the global nitrogen cycle are most intense in developed countries and in Asia, where vehicle emissions andindustrial agriculture are highest.[56]
Generation of Nr,reactive nitrogen, has increased over 10 fold in the past century due to globalindustrialisation.[2][57] This form of nitrogen follows a cascade through thebiosphere via a variety of mechanisms, and is accumulating as the rate of its generation is greater than the rate ofdenitrification.[58] Nr burial in lakes and oceans has been increasing in tandem with anthropogenic input, now double the Nr burial flux pre-industrial revolution. Reactive nitrogen can be denitrified in water or buried in sediments to accumulate. This buried Nr lies latent until its sediments are disturbed through events likestorms orfloods, when large amounts of nitrogen are reintroduced to the water where it can be denitrified and impact the environment.[59]
Nitrous oxide (N
2O) has risen in the atmosphere as a result of agricultural fertilization, biomass burning, cattle and feedlots, and industrial sources.[60]N
2O has deleterious effects in thestratosphere, where it breaks down and acts as acatalyst in the destruction of atmosphericozone. Nitrous oxide is also agreenhouse gas and is currently the third largest contributor toglobal warming, aftercarbon dioxide andmethane. While not as abundant in the atmosphere as carbon dioxide, it is, for an equivalent mass, nearly 300 times more potent in its ability to warm the planet.[61]
Ammonia (NH
3) in the atmosphere has tripled as the result of human activities. It is a reactant in the atmosphere, where it acts as anaerosol, decreasing air quality and clinging to water droplets, eventually resulting innitric acid (HNO3) that producesacid rain. Atmospheric ammonia and nitric acid also damage respiratory systems.
The very high temperature of lightning naturally produces small amounts ofNO
x,NH
3, andHNO
3, but high-temperaturecombustion has contributed to a 6- or 7-fold increase in the flux ofNO
x to the atmosphere. Its production is a function of combustion temperature - the higher the temperature, the moreNO
x is produced.Fossil fuel combustion is a primary contributor, but so are biofuels and even the burning of hydrogen. However, the rate that hydrogen is directly injected into the combustion chambers of internal combustion engines can be controlled to prevent the higher combustion temperatures that produceNO
x.
Ammonia and nitrous oxides actively alteratmospheric chemistry. They are precursors oftropospheric (lower atmosphere) ozone production, which contributes tosmog andacid rain, damagesplants and increases nitrogen inputs to ecosystems.Ecosystem processes can increase withnitrogen fertilization, butanthropogenic input can also result in nitrogen saturation, which weakens productivity and can damage the health of plants, animals, fish, and humans.[38]
Decreases inbiodiversity can also result if higher nitrogen availability increases nitrogen-demanding grasses, causing a degradation of nitrogen-poor, species-diverseheathlands.[62]
Increasing levels ofnitrogen deposition is shown to have several adverse effects on both terrestrial andaquatic ecosystems.[63][64] Nitrogen gases andaerosols can be directly toxic to certain plant species, affecting the aboveground physiology and growth of plants near largepoint sources of nitrogen pollution. Changes to plant species may also occur as nitrogen compound accumulation increases availability in a given ecosystem, eventually changing the species composition, plant diversity, and nitrogen cycling. Ammonia and ammonium – two reduced forms of nitrogen – can be detrimental over time due to increased toxicity toward sensitive species of plants,[65] particularly those that are accustomed to using nitrate as their source of nitrogen, causing poor development of their roots and shoots. Increased nitrogen deposition also leads to soil acidification, which increases base cation leaching in the soil and amounts ofaluminum and other potentially toxic metals, along with decreasing the amount ofnitrification occurring and increasing plant-derived litter. Due to the ongoing changes caused by high nitrogen deposition, an environment's susceptibility to ecological stress and disturbance – such as pests andpathogens – may increase, thus making it less resilient to situations that otherwise would have little impact on its long-term vitality.
Additional risks posed by increased availability of inorganic nitrogen in aquatic ecosystems include water acidification;eutrophication of fresh and saltwater systems; and toxicity issues for animals, including humans.[66] Eutrophication often leads to lower dissolved oxygen levels in the water column, including hypoxic and anoxic conditions, which can cause death of aquatic fauna. Relatively sessile benthos, or bottom-dwelling creatures, are particularly vulnerable because of their lack of mobility, though large fish kills are not uncommon. Oceanicdead zones near the mouth of the Mississippi in theGulf of Mexico are a well-known example ofalgal bloom-inducedhypoxia.[67][68] Even though there have been some efforts at reducing Nitrogen agricultural runoff, there has been no significant reduction in dead zone size.[69] The New YorkAdirondack Lakes,Catskills,Hudson Highlands,Rensselaer Plateau and parts ofLong Island display the impact of nitricacid rain deposition, resulting in the killing of fish and many other aquatic species.[70]
Freshwater has a lower ability to neutralize acidity, so acidification can occur with less nitrogen deposition.[71] This acidification can negatively impactfish and aquaticinvertebrates[72] while favoring phytoplankton that can handle more acidic environments.[73]
Ammonia (NH
3) is highly toxic to fish, and the level of ammonia discharged fromwastewater treatment facilities must be closely monitored. Nitrification viaaeration before discharge is often desirable to prevent fish deaths. Land application can be an attractive alternative to aeration.
Leakage ofNr (reactive nitrogen) from human activities can cause nitrate accumulation in the natural water environment, which can create harmful impacts on human health. Excessive use of N-fertilizer in agriculture has been a significant source of nitrate pollution in groundwater and surface water.[74][75] Due to its high solubility and low retention by soil, nitrate can easily escape from the subsoil layer to the groundwater, causing nitrate pollution. Some othernon-point sources for nitrate pollution in groundwater originate from livestock feeding, animal and human contamination, and municipal and industrial waste. Since groundwater often serves as the primary domestic water supply, nitrate pollution can be extended from groundwater to surface and drinking water duringpotable water production, especially for small community water supplies, where poorly regulated and unsanitary waters are used.[76]
TheWHO standard fordrinking water is 50 mgNO−3 L−1 for short-term exposure, and for 3 mgNO−3 L−1 chronic effects.[77] Once it enters the human body, nitrate can react with organic compounds throughnitrosation reactions in thestomach to formnitrosamines andnitrosamides, which are involved in some types of cancers (e.g.,oral cancer andgastric cancer).[78]
Human activities have also dramatically altered the global nitrogen cycle by producing nitrogenous gases associated with global atmospheric nitrogen pollution. There are multiple sources of atmosphericreactive nitrogen (Nr) fluxes. Agricultural sources of reactive nitrogen can produce atmospheric emission ofammonia (NH3),nitrogen oxides (NO
x) andnitrous oxide (N
2O). Combustion processes in energy production, transportation, and industry can also form new reactive nitrogen via the emission ofNO
x, an unintentional waste product. When those reactive nitrogens are released into the lower atmosphere, they can induce the formation of smog,particulate matter, and aerosols, all of which are major contributors to adverse health effects on human health from air pollution.[79] In the atmosphere,NO
2 can beoxidized tonitric acid (HNO
3), and it can further react withNH
3 to formammonium nitrate (NH4NO3), which facilitates the formation of particulate nitrate. Moreover,NH
3 can react with other acid gases (sulfuric andhydrochloric acids) to form ammonium-containing particles, which are the precursors for the secondaryorganicaerosol particles inphotochemical smog.[80]
Material was copied from this source, which is available under aCreative Commons Attribution 4.0 International License. Material was copied from this source, which is available under aCreative Commons Attribution 4.0 International License.
{{cite web}}: CS1 maint: multiple names: authors list (link){{cite book}}: CS1 maint: multiple names: authors list (link)| Cycles | |
|---|---|
| Research groups | |
| Related topics | |
| International | |
|---|---|
| National | |
| Other | |
